What happens when you play a movie's 5.1 sound source on 2.1 speakers? Will the sound from the 3 removed channels be truncated or added to the 2 channels left? Will it be EXACTLY the same as playing a 2.1 sound source from the same movie?
Like welshmousepk said, it depends on your setup. It also depends on your source and on you. Some DVDs have more than one audio scheme recorded on them, and you can go into the disc menu and select 2.1, or stereo. If my receiver, for example, gets a stereo signal, it will split out the low frequencies on its own in initiative, effectively making 2.1 out of stereo. I believe that this is normal behavior for a modern receiver. Some DVD players will downmix before sending the signal to the receiver if you tell them to and will create a 2.1 track out of the 5.1 source on the fly, mixing the surround information into the FL and FR channels according to some insanely scientifical formula that we need not worry our heads over. If you are playing DVDs on your computer and only have 2 speakers and a subwoofer connected, it will depend on what your sound card can do, but I would be surprised to find that the manufacturers have done anything like that. They figure you'll have all the speakers, and you'll get something bizarre, with the center channel missing.